1 /* InheritableThreadLocal -- a ThreadLocal which inherits values across threads
2 Copyright (C) 2000, 2001, 2002, 2003, 2004, 2005, 2006 Free Software Foundation, Inc.
4 This file is part of GNU Classpath.
6 GNU Classpath is free software; you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation; either version 2, or (at your option)
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12 WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
14 General Public License for more details.
16 You should have received a copy of the GNU General Public License
17 along with GNU Classpath; see the file COPYING. If not, write to the
18 Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
21 Linking this library statically or dynamically with other modules is
22 making a combined work based on this library. Thus, the terms and
23 conditions of the GNU General Public License cover the whole
26 As a special exception, the copyright holders of this library give you
27 permission to link this library with independent modules to produce an
28 executable, regardless of the license terms of these independent
29 modules, and to copy and distribute the resulting executable under
30 terms of your choice, provided that you also meet, for each linked
31 independent module, the terms and conditions of the license of that
32 module. An independent module is a module which is not derived from
33 or based on this library. If you modify this library, you may extend
34 this exception to your version of the library, but you are not
35 obligated to do so. If you do not wish to do so, delete this
36 exception statement from your version. */
40 import gnu.java.util.WeakIdentityHashMap;
42 import java.util.Iterator;
45 * A ThreadLocal whose value is inherited by child Threads. The value of the
46 * InheritableThreadLocal associated with the (parent) Thread is copied to
47 * the new (child) Thread at the moment of creation.
49 * <p>It is possible to make the value associated with the child Thread a
50 * function of the value that is associated with the parent Thread by
51 * overriding the <code>childValue()</code> method. The utility of this class
52 * is in transferring items like User ID or Transaction ID across threads
55 * @author Mark Wielaard (mark@klomp.org)
56 * @author Eric Blake (ebb9@email.byu.edu)
57 * @author Tom Tromey (tromey@redhat.com)
58 * @author Andrew John Hughes (gnu_andrew@member.fsf.org)
61 * @status updated to 1.4
63 public class InheritableThreadLocal<T> extends ThreadLocal<T>
67 * Creates a new InheritableThreadLocal that has no values associated
70 public InheritableThreadLocal()
75 * Determines the value associated with a newly created child Thread as a
76 * function of the value associated with the currently executing (parent)
77 * Thread. The default implementation just returns the parentValue.
79 * @param parentValue the value of this object in the parent thread at
80 * the moment of creation of the child
81 * @return the initial value for the child thread
83 protected T childValue(T parentValue)
89 * Generates the childValues of all <code>InheritableThreadLocal</code>s
90 * that are in the heritage of the current Thread for the newly created
91 * childThread. Should be called from the constructor Thread.
93 * @param childThread the newly created thread, to inherit from this thread
94 * @see Thread#Thread(ThreadGroup, Runnable, String)
96 static void newChildThread(Thread childThread)
98 // The currentThread is the parent of the new thread.
99 Thread parentThread = Thread.currentThread();
100 if (parentThread.locals != null)
102 Iterator keys = parentThread.locals.keySet().iterator();
103 while (keys.hasNext())
105 Object key = keys.next();
106 if (key instanceof InheritableThreadLocal)
108 InheritableThreadLocal local = (InheritableThreadLocal)key;
109 Object parentValue = parentThread.locals.get(key);
110 Object childValue = local.childValue(parentValue == sentinel
111 ? null : parentValue);
112 if (childThread.locals == null)
113 childThread.locals = new WeakIdentityHashMap();
114 childThread.locals.put(key, (childValue == null
115 ? sentinel : childValue));